Predict Your Race Time: Riegel, VDOT and 2.2 Rule

What a prediction needs to make sense

Any method is only as good as the data you give it. Before calculating, check four things:

• That the result is recent, from a time when you were in similar shape to today.
• That you ran it at maximum effort, on flat ground and without extreme conditions. A bad race or a course with a lot of climbing gives a result worse than the real one.
• That the reference distance is not very short: with results under 3 km, the prediction loses its meaning.
• That you have trained for the distance you want to predict. The methods do not know whether you have done long runs.

Method 1: the Riegel formula

Peter Riegel published this formula in 1977 and expanded it in 1981. It is a single calculation: new time = reference time x (new distance / reference distance) raised to 1.06. Its exponent applies to races lasting between 3.5 and 230 minutes.

For the most common jumps, the calculation boils down to multiplying by a fixed factor:

An example with a 10K in 55:00, which is 3,300 seconds. For the half marathon: 3,300 x 2.206 is about 7,281 seconds, that is 2:01:21, at 5:45 per km.

Method 2: VDOT

VDOT is a performance value used to compare races over different distances. It is not a lab measurement: it resembles maximal oxygen uptake, but it is calculated from your race time. The calculation is based on the equations published by Jack Daniels and Jimmy Gilbert in 1979.

It works in four steps:

1. You calculate your average speed in meters per minute.
2. You calculate the oxygen it costs to run at that speed.
3. You calculate what fraction of your maximum you can sustain for that long, which is smaller the longer the race.
4. You divide step 2 by step 3. That result is your VDOT.

With the same 10K in 55:00:

• Speed: 10,000 meters divided by 55 minutes is 181.8 meters per minute.
• Oxygen needed at that speed: 31.98.
• Fraction sustainable for 55 minutes: 0.894.
• VDOT: 31.98 / 0.894 = 35.8.

Then you look up which half marathon time gives that same VDOT: 2:01:56, at 5:47 per km.

Besides equivalent times, VDOT is used to calculate training paces. Runi's VDOT calculator gives them for VDOT values between 30 and 60.

Method 3: the 2.2 rule

It consists of multiplying your 10K time by 2.2 to estimate the half marathon. With 55:00, the calculation is 55:00 x 2.2 = 121:00, that is 2:01:00, at 5:44 per km.

It is not an independent method. The 2.2 is almost the Riegel factor for that jump (2.206) rounded, which is why the two calculations come out so close: in the example, 21 seconds apart. It only works for going from 10K to the half marathon.

The same runner with all three methods

Notice two things. In the half marathon, there are 56 seconds between the fastest and the slowest result. In the marathon, Riegel and VDOT are already 1:25 apart.

That gap grows as the jump gets bigger. With another runner, with a 5K in 28:30 (5:42 per km), the half marathon comes out at 2:11:06 with Riegel and 2:11:15 with VDOT, only 9 seconds apart. But the marathon comes out at 4:33:21 with Riegel and 4:30:02 with VDOT, which is a 3:19 difference.

So the methods resemble each other over nearby distances and drift apart as you lengthen the jump. And even if they agreed, none of them knows whether you have done long runs or what race day will be like.

When not to trust a prediction

• If you have not trained the new distance. Equivalent times over long distances tend to be optimistic if you have not trained for them, and without long runs the real time would be worse.
• If your result comes from an exceptional day, a course with a lot of climbing or a distance under 3 km.
• If you jump to a marathon from a 5K or a 10K: it is the prediction with the most error.
• If the result is old and your fitness has changed.

How to use a prediction: as a range, not a single number

1. Calculate with the method you prefer, or with several.
2. Turn the result into an average pace per km: total time divided by the distance.
3. Keep the range. In the half marathon example, from 5:44 to 5:47 per km.
4. On race day, start at the slow end of the range and adjust to how you feel.
5. Recalculate when you have a new result.

If it is your first 10K, how long will your 10K take has a full example with Riegel and a range between two references. And if you already have a specific goal, 10K in 50 minutes develops it with splits and strategy. And for a half marathon in 2:00:00, there is half marathon in 2 hours: splits and strategy.

Which method suits you

• If you want a quick calculation by hand, Riegel or the 2.2 rule.
• If you also want training paces, VDOT. With a recent result, in training zones you can calculate your paces and heart rates.
• If you do not want to do any maths, the running pace calculator and time predictor does it for you.

If you want to see the math applied to a specific distance, there is how long will your 5K take, how to run a 5K in 30 minutes, how to run a 10K in 60 minutes and how to run a 10K in 45 minutes. And to check every few weeks whether your result is improving, how to measure your running progress.

Common mistakes

• Using a bad race or a training run as the reference result.
• Applying the 2.2 rule to other distance jumps.
• Comparing several methods and keeping the fastest one.
• Taking a marathon prediction as a fixed goal.
• Not recalculating after a new result.